1. Field of the Invention
This invention relates to a membrane and a method used for producing syngas and, more particularly, to an improved ceramic membrane and method for converting methane to syngas.
2. Background of the Invention
Conversion of natural gas (mostly methane) to more valuable liquid products such as transportation fuels and chemicals is driven by the abundance of the feedstock, particularly in remote areas. For example, many remote natural gas fields throughout the world remain capped because there is no way to economically market the natural gas contained there. As such only 3 to 4 percent of natural gas feedstocks are utilized. The conventional route is to transport the natural gas by either pipeline or as liquefied natural gas. But pipelines are economical only when the gas field is close to the consumer. Liquefied natural gas can be considered only on a commercial scale.
The other alternative to exploiting natural gas reserves is to convert natural gas by oxidation through direct or indirect means. Direct conversion results in a partial oxidation of natural gas to methanol, formaldehyde or other organic compounds. However, this direct method produces products that are more reactive than the starting material.
Indirect approaches of methane conversion include the production of syngas (CO+H.sub.2). This conversion is depicted in Equation 1, below: ##EQU1##
Typical oxidation methods involve the conversion of methane to syngas by steam. The syngas is then used as a feedstock to produce more complex molecules via Fischer-Tropsch technology or methanol synthesis. These efforts typically are endothermic and also yield low conversion rates and selectivities.
Direct partial oxidation of methane with air as the oxygen source also has been attempted. However, inasmuch as downstream processing requirements cannot tolerate nitrogen, the need for oxygen separation technology arises, with its concomitant costs.
Efforts have been made using oxygen-transport materials to supply oxygen to organic streams to facilitate oxidation reactions. For example, U.S. Pat. No. 5,356,728 to Balachandran et al., discloses a reactor comprising a new material (whereby the new material has a chemical formula SrFeCo.sub.0.5 O.sub.x with x approximately equal to 3) and which is used to facilitate oxygen transport by exploiting the electron-and oxygen-transfer characteristics of the structure.
U.S. Pat. Nos. 5,639,437 and 5,580,497, also to Balachandran et al., describe an oxygen ion-conducting dense ceramic membrane (having the same chemical formula as stated in the previous paragraph) and methods for preparing ceramic membranes to facilitate oxidation reactions in a feed gas containing organic compounds.
However, the typical oxygen-transport materials such as those based on current Sr--Fe--Co systems lack certain mechanical properties required for long-term performance. Generally, these materials lack the superior mechanical properties such as strength and fracture toughness and micro-structural stability required in commercial applications, particularly those applications dealing with a highly reducing environment on one side of a membrane and an oxidizing environment on the other side of the membrane.
A need exists in the art for a membrane material which can facilitate oxygen transport to a compound stream while exhibiting durability in commercial and industrial scenarios. The membrane should withstand multiple thermal cyclings without a decrease in oxygen transport characteristics. The manufacture of the membrane should be relatively economical and use materials widely available.